Have you looked at the source for that 41 million Spanish speakers? It's available here:
https://archive.ph/20200214011034/https://factfinder.census....Of that 41 million, 24.6 million "Speak English only or speak English 'very well', which is 58.9%. Of those under 18, that fraction goes up to 81.1%
For comparison, other Indo-European languages has 69.3% and 81.5% for those under 18. Asian and Pacific Island languages: 54.1%/75.4% Other languages: 69.0%/78.7%
It's not a perfect proxy for the claim about the loss of language with each generation, but it's a good approximation. The pattern is similar across all languages (and the difference between Spanish and other Indo-European languages is negligible). I've read about research that has more explicitly found that there is no unique pattern for Spanish in the U.S.
What you're seeing is not an indication that Spanish has a different pattern, but that there are more Spanish-speaking immigrants and are a more visible immigrant community to outsiders. My great-grandparents settled into communities of Czech and Slovene speakers and had children who were fully bilingual while of my parents' generation, only my father has any facility with Czech, which his mother credited to his being effectively raised by his grandmother in his early years. My own Czech knowledge doesn't extend much behind, Promiňte, prosim, rozumite anglický?
You can find communities where the first language is Chinese in almost any sizable urban area. There are parts of Chicago where the first language is Polish, but as the community is smaller and less visible to outsiders, most people don't even realize it. Los Angeles has numerous linguistic enclaves. When Pope John Paul II visited Los Angeles, what most excited the pope was that the Archdiocese offered Mass in more languages than any other diocese in the world. (Non-English church services are a big indicator of these linguistic communities).